Efficiency explained

Efficient. It's a relatively simple word, nine letters and four syllables long. You see it everywhere, these days, in advertisements, news stories and other commercials. To some, "efficient" means getting the most out of something. To others, it means stuffing a lot of things into a small car or driving a long distance on a gallon of gasoline. To others, it is a form of magic that makes everything "better," if "they" ("big business", "the government" or whomever, pick your villain) would only allow "it" (pick your rumored invention) to be sold.

It's thrown around a great deal, but what does it really mean?

To a scientist or an engineer, "efficiency" is one thing -- a ratio of how much you get out of a system divided by how much you put in. The result is always a decimal or a percentage and never goes over 100%. Here's an example:

Let's say you are in charge of the town water service and you put 100 gallons of water into the water main. Ideally, you would get every last water molecule of that 100 gallons out at the other end. That would be 100% efficient.

Unfortunately, the real world is rarely that way. Pipes crack, letting water leak into the surrounding soil. Occasionally burst, sending water a fountain of water spraying all over the place, leaving everyone down stream with no water at all. So if you put 100 gallons into your theoretical water main and 25 gallons leaks away, then you have 75 gallons coming out the other end of the pipe and since 75 divided by 100 is 0.75 -- or 75% -- your pipe is "75 percent efficient."

Now what about the claims that some things are "200 percent efficient?" Is this true, or is it just hopes and wishes? Here, you have to be very careful what you say and how you hear what it said. As I said, you cannot make something more than 100% efficient -- to be more efficient than that is like pouring a quart of water out of a pint bottle. However, you can increase efficiency by more than a factor of 100% if you start with something that was not very efficient in the first place.

Getting back to our leaky water main, let's say the pipe burst, sending 90 gallons spraying into the air. What you'd get out is ten gallons of the original 100, making your system 10% efficient. Then a work crew replaces the broken section of pipe with a temporary patch that only leaks 50 gallons. In doing so, they have made the pipe 400% more efficient than it originally was -- but it is now still only 50% efficient compared to the original 100 gallons of water that were put into it.

The same thing can be said of a car's engine. Most of the energy in a gallon of gasoline is wasted in one way or another. Some of it escapes in the form of heat, through friction, the radiator and so on. Some of it disappears through "Pumping Losses" -- which is a fancy way of saying "the effort needed to suck air into the cylinder, push it out again, and shuffle it around in the crank case as the pistons go up and down." And some of it goes in through the intake valve and back out the exhaust valve without ever being burned. Your car's catalytic converter is there to take care of this unburned fuel.

So what about wonder-widgets like the "200 percent efficient" carburettor? That is an urban legend that has been around for a long, long time. If this thing is supposed to make an engine "200% efficient" in absolute terms, it can't happen. As I said, that would be like making a pint's worth of gasoline do the work of a gallon of the stuff.

On the other hand, if the carburettor improves the fuel-air mixture so that more of the fuel is burned, then the engine can go from, say, 12% efficient (sending 88% of the gasoline's energy out the tail pipe, about what engines of the 1950s wasted) to 24% efficient (which is pretty good, even these days). In that case, the engine is twice as efficient, or "200% efficient" when compared with the way it was before being modified. But it is still only 24% efficient in absolute terms. Like the crew fixing the broken water main, you've helped the situation, but you haven't made it ideal. The same holds true of everything else that uses energy in one way or another.

Ultimately, you have to take talk of 'increased efficiency" claims with a grain or two of salt. If someone makes a grandiose claim, ask him to explain what, exactly, he means by "200% efficient." If he gives you a straight answer of how much improvement his wonder-widget made and still allows that some energy is being wasted, you can believe him. On the other hand, if he's selling a pint bottle and claiming it will hold a gallon of stuff...you might want to look elsewhere.

You should ask the same question regarding claims by car companies that their car is "more fuel efficient" than someone else's. Fuel mileage claims are based on how much pollution is produced during the Environmental Protection Agency's city and highway simulation tests. Most of the time, these figures will give you a rough idea how much fuel a car uses, but not necessarily how efficient it is with the energy in a gallon of gasoline. As the saying goes, "size matters," but so do a number of other things.

While it is true that a smaller engine uses less fuel because it takes in less air/fuel mixture with each revolution of the crank shaft, that doesn't necessarily mean it's more efficient than a larger one. Here's an example.

In 1969, Sports Car Graphic magazine tested the Chevrolet Camaro Z-28. That car had a five liter (302 cubic inch) V-8 engine with a four barrel carburettor, a four speed transmission. Its engine developed 190 horsepower and the 3,000 pound car topped out at around 120 miles an hour and got around 13 miles per gallon, which was about par for the course, then. According to Road&Track magazine, today's Camaro SS has 6.2-liter (376 cu. in.)engine, weighs closer to 4,000 pounds. At one point, one of the test drivers floored it on an empty road in Nevada and saw 162 miles per hour on the speedometer, That car generated 426 bhp (more than twice as much as the older car), yet it got 20 to 25 miles per gallon in average driving.

Admitted, other things are involved in fuel mileage -- aerodynamic drag, gear ratios and a lot of other things -- but the new car's engine is undeniably more efficient than the old one. It's not "200% efficient," by any means, but it's doing right well for itself, for what it is.

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John Lloyd Is a long-time observer of the sports car scene and its technical developments. He has three college degrees in liberal studies, electronics and drafting. He also has "my archives" -- 40 years worth of automobile magazines. He combines this with an accute understanding of things technical and a long knowledge of automotive history that gives him an understanding of how cars work, where they came from and the difference between "greatness" and "it seemed like a good idea at the time.